1. Technical Field of the Invention
The present invention relates generally to directional couplers, and specifically to high-power directional couplers.
2. Description of Related Art
A directional coupler has a through arm through which a signal passes and at least one coupled arm that samples the signal. At a basic level, a high-power directional coupler causes a sample of an electromagnetic wave propagating on the through arm to propagate on the coupled arm. Therefore, the coupled arm serves to sample the signal on the through arm. A directional coupler is capable of sampling signals propagating in two different directions. A signal flowing in a first direction on the through arm is sampled on one port of the coupled arm, while a signal flowing in the opposite direction is sampled on the other port of the coupled arm.
To measure output power or other high-power signals in a system, high-power handling capability is desirable for dual directional couplers. For example, dual directional couplers with high-power handling capabilities are well-suited to measure the output power of a base station within a cellular network. High-power directional couplers are also well-suited to measure the return loss of base station antennas by measuring both the forward power, which propagates from the base station to the antenna, and also the reverse power, which is reflected from the antenna and propagates in the opposite direction.
Traditionally, high-power directional couplers have been constructed of a number of machined metal parts. An extensive amount of labor is usually involved in assembling the large number of machined metal parts required for such high-power directional couplers. The machined metal parts taken in conjunction with the amount of labor have resulted in expensive high-power couplers on the order of several hundred dollars.
Furthermore, the final tolerances for the geometry of the coupled arms in traditional machined metal high-power couplers are relatively loose, due to the large number of separate machining and assembly steps. The resulting loose tolerances have produced relatively large performance variations amongst traditional high-power directional couplers. Therefore, many of the traditional high-power directional couplers have provided tuning slugs that can be adjusted to produce the required coupler performance. The inclusion of a tuning slug further increases the cost of these traditional machined metal high-power couplers.
High-power directional couplers are especially expensive when compared with low power couplers. Low power couplers are commonly fabricated on a dielectric printed circuit board either as microstrip or stripline designs. Microstrip coupler designs have metal plated on both the top and bottom of dielectric printed circuit board, with the top forming the arms and the bottom forming the ground plane. Stripline coupler designs have the metal arms xe2x80x9csandwichedxe2x80x9d in the middle of the dielectric printed circuit board, with metal grounds on both the top and bottom of the dielectric printed circuit board. As is well known in the art, printed circuit board dielectric material costs much less than the machined metal parts and tuning slugs used in high-power couplers. In addition, fabricating couplers on printed circuit boards produces more repeatable couplers with improved performance characteristics. Therefore, coupler designers have previously considered using printed circuit board dielectric material to fabricate high-power couplers.
However, it has not proved practical to construct high-power directional couplers on printed circuit boards in microstrip or stripline configurations due to the insertion loss in the dielectric printed circuit board material. In these type of printed circuit board structures (microstrip or stripline), electric and magnetic fields necessarily penetrate the dielectric material of the printed circuit board. The dielectric material typically has inherent losses, which increases the insertion loss of the coupler. In addition, as the dielectric constant of the dielectric printed circuit board material becomes higher than that of air, the transmission lines of the through and coupled arms of the coupler become correspondingly narrowed, further increasing the insertion loss of the coupler. Therefore, in the past, printed circuit board designs have been inappropriate for high-power couplers. What is needed is a reduced cost, high-power directional coupler with improved performance characteristics as compared with machined metal high-power directional couplers.
The present invention provides a high-power directional coupler formed from a substrate, such as a printed circuit board formed of a dielectric material. The through arm and coupled arm(s) of the coupler have a conductor, typically metal, plated on the top, bottom and edges of the dielectric material. Thin non-conductive struts of the dielectric material interconnect the separate arms of the coupler. A metal package surrounding the coupler forms the outer ground.
Since the through arm has metal plated onto the edges, as well as the top and the bottom, the dielectric material is completely enclosed in metal. As a result, the RF fields flow primarily on the metal (outside of the dielectric), and generally do not penetrate into the dielectric. Therefore, the performance of the coupler is independent of the dielectric material""s properties (e.g., loss tangent, dielectric constant, etc.). Thus, the coupler has a low insertion loss, enabling the coupler to operate at high-power. In addition, variations in the dielectric material""s properties do not effect the performance of the coupler. Therefore, the coupler is more repeatable with improved performance characteristics. Furthermore, the independence of the dielectric material with respect to the coupler performance allows an inexpensive dielectric, e.g., FR4, to be used.
As a further advantage, fabricating high-power directional couplers out of printed circuit board material is inexpensive, compared to fabricating high-power directional couplers out of machined metal parts. For example, machined metal high-power directional couplers typically require expensive connectors and cables to connect to additional circuitry, whereas high-power directional couplers fabricating from printed circuit board material can be easily integrated with microstrip circuitry, such as switches and resistors. In addition, the number of assembly parts and processing steps are reduced by using the thin non-conductive struts of the dielectric to interconnect the separate arms of the coupler. The reduced processing steps further minimizes tolerances in fabrication geometry, and leads to more repeatable performance with no manual alignments. Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings.